GENERAL ANATOMY. 



Besides the inner membrane, there exists also in the sub- 

 stance of the valves, dense cellular tissue, and sometimes dis- 

 tinct fibres; sometimes they are areolar and perforated like 

 lace. In the veins or sinuses of the dura mater, there occur 

 only some transverse fibres, which may be considered as ru- 

 dimentary valves. 



The valves are generally disposed in pairs placed alternative- 

 ly, according to two opposite diameters of the vein. 



They are three, and three in the great veins, as in the crural 

 and iliac; they are seldom observed quadruple, and very sel- 

 dom or never quintuple. In twigs, with a diameter of half a 

 line and less, they are single. 



There are by no means valves at all the places where a twig 

 unites to a branch, or where a branch opens into a trunk; nor 

 are they everywhere at the same distance; they are nowhere 

 closer to each other than in the smallest veins. Valves are 

 found in the veins of the extremities, more in the subcutane- 

 ous than in the deep seated ones, in those of the face, neck, 

 tongue and tonsils, at the termination of the cardiac vein, in 

 the tegumentary veins of the abdomen, in those of the testi- 

 cles, penis and clitoris, in the internal and external iliac veins, 

 sometimes in the renal veins, and rarely in the vena azygos. 



There are none in the encephalic, spinal and diploic veins, 

 in those of the lungs, in the vena porta, in the umbilical vein, 

 in the vense cavaB, if we except those at the entrance of the 

 vena azygos, in the uterine veins, and in the median vein. 



There are generally a great many valves in the superficial 

 veins, fewer in the deep seated or intermuscular veins, and 

 still fewer in the veins of the splanchnic cavities; they are 

 numerous in the most depending parts, and therefore in the 

 lower extremities, less so in the upper, and still less numer- 

 ous in the head and neck. 



The valves are applied against the parietes of the veins, 

 when the course of the blood is free and easy, but when it 

 meets with obstacles in its passage, the valves separate from 

 the walls, close the vein, sustain the blood, and prevent its 

 reflux towards the capillary vessels. 



